Optical transmission system using in-line amplifiers

ABSTRACT

In a system connecting a transmitter and a receiver using transmission paths and repeaters (in-line amplifiers), red chirping whose α parameter is positive is performed for an optical signal on a transmitting side. Each of the repeaters includes a dispersion-compensator for compensating the amount of dispersion on a preceding transmission path. The amount of dispersion compensation of the dispersion-compensator included in the transmitter is made constant. The dispersion-compensator included in the receiver is arranged in order to compensate the amount of dispersion on a preceding transmission path. A spread of a pulse width on a transmission path can be efficiently compensated by using the compensation capability of the dispersion-compensators and the red chirping on the transmitting side.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of application Ser. No.08/929,090, filed Sep. 15, 1997, now allowed.

BACKGROUND OF THE INVENTION

[0002] 2. Field of the Invention

[0003] The present invention relates to an optical transmission systemusing fibers, and more particularly to an optical transmission systemusing in-line amplifiers.

[0004] 2. Description of the Related Art

[0005] An optical transmission system is now being developed withincreased capacity and an extended span of transmission. An increase ofa bit rate and wavelength division multiplexing system are now beingdiscussed so as to increase capacity. In the meantime, an opticalamplifier has been introduced so as to extend the span of transmission.The types of optical amplifiers include a post-amplifier (forstrengthening output of transmission power), a pre-amplifier (forincreasing the sensitivity of reception power), and a repeater (in-lineamplifier). The optical amplifier is currently under development at aproduction level. The introduction of the optical amplifier allows thedifference between optical intensities at transmission and reception tobe extended, and an allowable loss in the fiber is increased.

[0006] A system configuration using a post-amplifier and a pre-amplifierhas been put into practical use. Additionally, the in-line amplifier isunder development in order to extend the reproduction relay interval.Here, the in-line amplifier is a repeater which amplifies an opticalsignal unchanged without converting it into an electric signal, andtransmits the amplified signal.

[0007] The system using in-line amplifiers, however, poses a new problemwhere amplified spontaneous emission lights, from plurality ofamplifiers, due to the connection of the plurality of amplifiers, areaccumulated, and the S/N ratio is lowered. The lowering of the S/N ratioleads to the degradation of a minimum reception power of a receiver. Toobtain a predetermined system gain in consideration of this degradation,transmission power output must be strong thereby a lower limit value ofthe transmission power is determined. Furthermore, if the transmissionpower output is stronger than a threshold (+8 dBm for a dispersionshifted fiber, and 10 dBm or more for a single mode fiber, although itdepends on the length of a transmission path or a wavelength), thewaveform is significantly degraded due to the non-linear effect of afiber. One type of wavelength degradation is an optical Kerr effect(refractive index changes depending on an optical intensity). This is aphenomenon where a frequency (wavelength) shift occurs at the rising andfalling edges of an optical signal pulse (SPM: Self-Phase Modulation).Even if the width of an optical wavelength is narrow beforetransmission, the width of the wavelength increases, and at the sametime, a reception waveform significantly changes due to the influence offiber dispersion. The upper limit of optical transmission power isdetermined in consideration of such an influence.

[0008] Fiber dispersion means that the speed of light propagating on afiber depends on its wavelength. An optical pulse having a certainwavelength width is widened or compressed after fiber propagation. Thiseffect is referred to as fiber chromatic dispersion. Accordingly, areception waveform in an optical transmission system after fiberpropagation varies depending on the chromatic dispersion, and atransmission error will occur depending on the degree of dispersion.Therefore, the fiber dispersion imposes a restriction on thetransmission distance.

[0009] With a system using an in-line amplifier which amplifies anoptical signal without conversion, such non-linear effect and dispersionare accumulated while the optical signal travels. Accordingly, itbecomes quite impossible to properly receive the optical signal on areceiving side unless suitable compensation is made.

[0010] In the meantime, a system implemented by combining blue chirpingon a transmitting side and dispersion compensation in repeaters and areceiver was conventionally proposed.

[0011]FIG. 1 is a schematic diagram showing a combination ofconventional pre-chirping and dispersion compensators.

[0012] In this figure, a transmitter 1000 and a receiver 1010 areconnected by transmission paths 1003, 1006 and 1009, and repeaters 1004and 1007. The transmitter 1000 is composed of an E/O 1001, forconverting an electric signal into an optical signal, and apost-amplifier 1002. The transmitter 1000 performs blue-chirping on theoptical signal, and transmits the signal. The transmitted optical signaltravels along the transmission path 1003 and enters the repeater 1004.The repeater 1004 amplifies the optical signal, and performs dispersioncompensation using the dispersion compensator 1005. The amount ofdispersion compensation is a constant value. The optical signal, whichis further amplified and dispersion-compensated, passes along thetransmission path 1006 and enters the repeater 1007. The repeater 1007also amplifies the signal, performs dispersion compensation andtransmits the signal on the transmission path 1009. The optical signalpasses through repeaters whose number is predetermined, until it reachesthe receiver 1010. The receiver 1010 amplifies the received opticalsignal using a pre-amplifier, performs dispersion compensation using adispersion-compensator 1012, inputs the signal to an O/E 1013 in orderto convert the optical signal into an electric signal, and extractsnecessary data.

[0013] That is, the conventional combination is implemented by combiningblue chirping (especially, chirping parameter=−1) as the pre-chirping,and compensation by the dispersion-compensators arranged in in-lineamplifiers and the receiver (between the pre-amplifier and the O/E). Ifthe blue-chirping is performed in a fiber of +dispersion, the outputpulse is compressed due to the characteristics of the fiber of+dispersion, and the chirping. As a result, a transmission distance ismade relatively longer. Especially, in a system which does not useoptical amplifiers, an optical signal having the wavelength of 1.5 μm ismore effective when it travels along a single mode fiber (1.3 μmzero-dispersion wavelength). Accordingly, dispersion compensationimplemented by combining the pre-chirping and the succeedingcompensation was considered also to be effective in a system usingoptical amplifiers. If the amount of dispersion compensation is set inorder to keep a residual dispersion value (obtained by subtracting theamount of dispersion compensation from a total amount of dispersion of atransmission fiber) constant, a stable transmission characteristic canbe obtained.

[0014] However, if output of the transmission power is increased byintroducing optical amplifiers according to this method, the influenceof the non-linear effect of an optical fiber remarkably appears. Theinfluence of the non-linear effect is equivalent to the characteristicof blue chirping. The pulse width of the transmission waveform isnarrowed due to the influence of the pre-chirping at the transmitter andthe non-linear effect of the optical fiber. As a result, the influenceof the non-linear effect remarkably appears, and the waveform issignificantly changed for the dispersion.

[0015] The problems posed by the method for performing blue-chirping atthe time of transmission are listed below.

[0016] 1) Output of transmission power cannot be increased.

[0017] 2) Dispersion-compensation on a transmitting side is ineffective.

[0018] 3) The dispersion-compensation is performed in in-line amplifiersand on a receiving side due to the ineffectivity on the transmittingside in consideration with 2). Accordingly, the losses ofdispersion-compensators become larger, and the tolerance of the lossesbecomes difficult as transmission distance is extended. Lowering thelevel of an optical input to the O/E leads to degradation of receptionsensitivity, and this imposes a limitation. Furthermore, optical inputpower may sometimes have an upper limit depending on thedispersion-compensator to be used.

[0019] 4) The tolerance of the amount of dispersion-compensation whichcan ensure the transmission characteristic is small.

[0020] 5) The number of types of different devices increases when thedispersion-compensators are prepared according to a transmissiondistance due to the small tolerance as a result of 4).

SUMMARY OF THE INVENTION

[0021] The object of the present invention is to provide a technologywhich can compensate for transmission degradation especially due tofiber dispersion, and ensure a good transmission characteristic for alonger distance, in an optical in-line amplifier system.

[0022] The optical transmission system according to the presentinvention assumes the use of repeaters (in-line amplifiers). Itcomprises a transmitter, repeaters, a receiver and transmission pathsfor interconnecting these modules. The present invention ischaracterized in that the transmitter performs chirping whose aparameter is positive for an optical signal and each of the repeatersand the receiver include a dispersion-compensator having an amount ofdispersion compensation to compensate for dispersion from a transmissionpath preceding each of the repeaters and the receiver.

[0023] Since the non-linear effect that the optical signal receives onthe transmission path corresponds to the blue chirping, this effect canbe compensated for by performing red chirping whose a parameter ispositive on the transmitting side. This leads to the effect ofpreventing the waveform of the optical signal from being degraded.

[0024] Furthermore, degradation of the optical signal can be preventedmore effectively by setting the amount of dispersion compensation inorder to compensate for the dispersion from a preceding transmissionpath, in each of the repeaters or the receiver.

[0025] With the above described configuration, an optical signal can betransmitted by performing red chirping to compensate for the non-lineareffect in order to prevent a waveform from being degraded even if anoptical output power is increased on the transmitting side.

[0026] Additionally, since the amount of dispersion compensation in arepeater or a receiver can be achieved from a combination of unitmodules, implementation as a product is relatively easy.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a schematic diagram showing a combination of aconventional pre-chirping and dispersion-compensators;

[0028]FIG. 2 is a schematic diagram showing the basic configuration ofan embodiment according to the present invention;

[0029]FIGS. 3A and 3B are schematic diagrams showing the dependency of a1R transmittable distance range corresponding to a change of anparameter;

[0030]FIGS. 4A and 4B exemplify a menu setting at the time ofpropagation along a single mode fiber;

[0031]FIG. 5 is a schematic diagram showing a dispersion compensationmethod and the degradation of a waveform on a receiving side when a 1Rinterval varies depending on a period at the time of the propagationalong a single mode fiber;

[0032]FIG. 6 is a graph showing the number of 1 Rs satisfying atransmission characteristic required for an amount of dispersioncompensation on a transmitting side, which is obtained for eachparameter;

[0033]FIG. 7 is a schematic diagram showing the relationship of a 1Rinterval to an amount of a 1R residual dispersion;

[0034]FIGS. 8A through 8D are schematic diagrams explaining unit modulesof a dispersion-compensator;

[0035]FIGS. 9A and 9B exemplify the structure of an optical switch foruse in a unit module of the dispersion-compensator; and

[0036]FIGS. 10A through 10C exemplify the structures for compensatingdispersion other than a dispersion-compensating fiber.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037]FIG. 2 is a schematic diagram showing the basic configuration ofan optical transmission system according to an embodiment of the presentinvention.

[0038] In this figure, a transmitter 1 and a receiver 7 are connected bytransmission paths 2, 4, 6, . . . and repeaters 3, 5, . . . . Thetransmitter 1 is composed of an E/O (Electric-to-optical signalconverter) 8, a dispersion-compensator 9 and a post-amplifier 10. TheE/O 8 is intended to convert an electric signal into an optical signal.The dispersion-compensator is intended to perform a predetermined amountof dispersion compensation on the side of the transmitter 1. Thepost-amplifier 10 is intended to amplify an optical output in order toallow the optical signal to be transmitted farther along a transmissionpath. In addition, red chirping whose a parameter ranges between 0 and+2 is performed on the side of the transmitter 1, according to thepresent invention. The amount of dispersion compensation of each of thedispersion-compensators 11 and 12 included in the repeaters 3 or 5 isadjusted in order to compensate for the dispersion from the precedingtransmission path (transmission path from the preceding repeater to thelocal repeater: the length of a transmission path between repeaters isreferred to as a 1R transmission distance or a 1R interval). That is,the dispersion-compensator 11 possesses the amount of dispersioncompensation necessary for compensating the dispersion from thetransmission path 2, while the dispersion-compensator 12 possesses theamount of dispersion compensation necessary for compensating thedispersion from the transmission path 4. Also dispersion-compensatorsarranged in other repeaters, (not shown in this figure) are configuredso that they possess the amount of dispersion compensation necessary forcompensating the dispersion from a preceding transmission path. Thereceiver 7 is composed of a pre-amplifier 13, a dispersion-compensator14, and an O/E (optical-to-electric signal converter) 15. Thepre-amplifier 13 is intended to amplify a transmitted optical signal sothat it can be detected with ease. The dispersion-compensator 14 isarranged in order to compensate for the dispersion from the transmissionpath preceding the receiver 7. The O/E 15 is intended to convert anoptical signal into an electric signal, and output the converted signalto a device for extracting data.

[0039] A transmitting side narrows the pulse width of the signal usingchirping and dispersion-compensation, and outputs the signal having anarrow pulse width to a transmission path (fiber). The signal isinfluenced by the non-linear effect of the fiber (narrowing of the pulsewidth) and the dispersion of the fiber (widening of the pulse width).Since the effects are cancelled out by each other only, a slightwaveform change is made from the dispersion. The degradation caused bythe dispersion is improved by performing the dispersion compensation ineach of the in-line amplifiers and on the receiving side. That is, thewaveform is improved (the pulse width is narrowed), and input to thereceiver.

[0040] One advantage of this compensation method is that the dispersioncompensation can be made effective on the transmitting side. This isrequired for narrowing the pulse width of a waveform to be transmitted.By narrowing the pulse width and transmitting the signal having anarrowed pulse width to a transmission path, the amount of inter-symbolinterference on one side of a logical value “0” is reduced. As a result,an improved transmission characteristic can be obtained. That is, theoptimization of the narrowing of the waveform becomes vital, and thedetermination of the amounts of transmission chirping and dispersioncompensation on the transmitting side depends on how to optimize thepulse width of the waveform.

[0041] Another advantage of this method is that the tolerance of theamount of dispersion compensation, which secures the desiredtransmission characteristic, can be increased. Since the pre-chirping ofa transmitter is the red chirping, the pulse width of a waveform iswidened at the time of propagation along an optical fiber of+dispersion. In the meantime, because the influence of the non-lineareffect in the optical fiber is equivalent to the characteristic of theblue chirping, the pulse width is narrowed. That is, the influence ofthe non-linear effect is cancelled out by the pre-chirping of thetransmitter. As a result, only a slight waveform change is made from thedispersion. Accordingly, the range of the transmission distance whichcan satisfy a required transmission characteristic is widened for a setamount of dispersion compensation. This leads to a reduction in thenumber of types of dispersion-compensators. That is, the most importantpoint of this method is to how to set the parameter.

[0042] In the system shown in FIG. 2, a transmission pulse is narrowedby combining the characteristics of the chirping parameter and thedispersion-compensator on the transmitting side, and is output to thetransmission path. Furthermore, the characteristic of the chirping atthe transmitter is cancelled out by the characteristic of the chirpingwhich occurs due to the influence of the non-linear effect. On thereceiving side, the waveform, degraded due to the dispersion, iscompensated (narrowing of the pulse width) by combining thecharacteristics of the chirping parameter and thedispersion-compensator.

[0043]FIGS. 3A and 3B are schematic diagrams showing the dependency ofthe range of a 1R transmittable distance the a parameter.

[0044] These schematic diagrams show the range of the 1R transmittabledistance range which can satisfy a required transmission characteristicfor each a parameter under the predetermined conditions, such as theamount of dispersion compensation and the number of periods. As shown inFIG. 3A, three repeaters 23, 24 and 25 are arranged between atransmitter 21 and a receiver 22. These repeaters are connected by thetransmission paths 26, 27, 28 and 29. FIG. 3B shows, for each aparameter, the range in which a required transmission characteristic canbe obtained, when the amount of dispersion compensation of each of thetransmitter 21, receiver 22 and the repeaters 23, 24 and 25 is set to aconstant value, and the 1R transmission interval is assumed to be aparameter.

[0045] As shown in FIG. 3B, a wide range of the 1R transmission distancecan be secured if the value of the a parameter is positive. Actually,the 1R transmission distance is short when the value of the a parameteris close to “0”. To cancel out the non-linear effect occurring on atransmission path by making an optical output stronger, it is effectiveif the a parameter is set to a positive value. Accordingly, the aparameter adopts the positive value. Furthermore, it is estimated fromthe result of FIG. 3B that the value of the a parameter in theneighborhood of “+1” is best. However, since this figure assumes thatthe transmission output is +14 dBm, the result is obtained based on thisassumption. If the transmission output is changed, the optimum value ofthe a parameter is considered to shift.

[0046] The transmission output in an in-line amplifier system iscurrently assumed to be of the order of +5 to +17 dBm. Therefore, thechange of the order of −9 to +3 dB for +14 dBm is considered. The amountof a shift of frequency at a light source is proportional to the aparameter, while the amount of a shift of frequency due to thenon-linear effect of a transmission path fiber is proportional to atransmission output when the transmission distance is fixed. Therefore,the optimum value of the a parameter is considered to vary in proportionto the amount of change in the transmission output according to thepresent invention, where transmitter chirping cancels the non-lineareffect.

[0047] Consequently, the optimum value of the a parameter is expected tochange with power change of −9 to +3 dB, from the optimum value for=+1,that is, in the range for from 0.125 to 2. The lower limit, however, isreplaced with “0” which is the lowest extreme in consideration of thecase in which optical amplifiers are not used, and the transmissionoutput level is low. Finally, the range from 0 to 2 is considered to bean effective range for the parameter.

[0048] Accordingly, the range of the 1R transmittable distance can besecured in a wide range where the value of the a parameter is positive.This allows a reduction of the number of types ofdispersion-compensators. Accordingly, it is effective that the aparameter is set within the positive range.

[0049] With the improvements on the conventional method summarizedaccording to the above description, the following points can be cited:—

[0050] 1) The tolerance of the amount of dispersion compensation, whichcan secure a transmission characteristic, increases.

[0051] 2) The number of device types can be reduced when type ofdispersion-compensator is set according to the transmission distance, asa result of 1).

[0052]FIGS. 4A and 4B exemplify a menu setting at the time ofpropagation along a single mode fiber.

[0053] As shown in FIG. 4A, three repeaters are set, and the amount ofcompensation is set so that the dispersion compensation can be made fora 1R interval range from 0 to 80 km. A dispersion-compensator isarranged in each of the transmitter 21, receiver 22, and the repeaters23, 24 and 25. The amount of dispersion compensation on the transmittingside is assumed to be −600 ps/nm, and the amount of dispersioncompensation within in-line amplifiers/on a receiving side is reviewed.

[0054]FIG. 4B shows the result of the review of the requiredcompensation at the in-line amplifiers/on the receiving side.

[0055] The shaded portion in FIG. 4B represents an allowable 1R intervalfor each amount of dispersion compensation. As shown in FIG. 4B, therange from 0 to approximately 22 km can be secured as a 1R transmissiondistance between in-line amplifiers, or between an in-line amplifier anda receiver, if the amount of dispersion compensation is 0 ps/nm. Tosecure the range of the 1R transmission distance exceeding approximately22 km, it is sufficient that the amount of dispersion compensationwithin an in-line amplifier or on a receiving side is set to −300 ps/nm.This allows a 1R transmission distance from approximately 22 to 38 km tobe covered. Similarly, the dispersion of a transmission path betweenin-line amplifiers or between an in-line amplifier and a receiver can becompensated by setting the amounts of dispersion compensation to −600ps/nm for the range from approximately 38 to 58 km to, −900 ps/nm forthe range from approximately 58 to 78 km, and to −1200 ps/nm for therange from approximately 78 to 80 km.

[0056] As described above, an optical transmission system which usesin-line amplifiers and prevents the waveform of an optical signal fromdegrading can be implemented by preparing five types 0, −300, −600, −900and −1200 ps/nm, of dispersion compensation, when the 1R interval is setat a range from 0 to 80 km.

[0057] In an actual system, the 1R interval may differ for eachinterval. Even in such a case, the amount of dispersion compensation canbe selected in order to obtain a required transmission characteristicwith this method. The present invention is characterized in that theamount of dispersion compensation is set depending on a distance priorto a repeater.

[0058]FIG. 5 shows the method for compensating dispersion and thedegradation of a waveform on a receiving side when a 1R interval differsfor each interval of propagation along a single mode fiber.

[0059] The amount of dispersion compensation on a transmitting side isassumed to be −600 ps/nm, and two methods for compensating dispersionwithin in-line amplifiers/on a receiving side are presented. The uppercompensation condition (1) is intended for a 3R transmission distance,and the amount of dispersion compensation within in-line amplifiers andon a receiving side is set to an identical value. The lower compensationcondition (2) is intended for the 1R transmission distance, and theamounts of dispersion compensation within an in-line amplifier and on areceiving side are separately set. FIG. 5 shows the equalized waveformsof the O/E.

[0060] Under the upper compensation condition (1) shown in FIG. 5, theamounts of dispersion compensation within the in-line amplifiers and ona receiving side are set to −600 ps/nm. Judging from the eye patternsobtained for the various patterns of the 1R interval, an eye opening ofa certain degree is obtained if the 1R interval is set to 80 and 10 kmin turn. However, since almost no opening is obtained in the othercases, it is nearly impossible to properly read the logical values “1”and “0”.

[0061] In the meantime, under the lower compensation condition (2), theamounts of dispersion compensation within an in-line amplifier and on areceiving side are set to 0 ps/nm if the 1R interval is 10 km, and to−1200 ps/nm if the 1R interval is 80 km, so that the amounts aresuitable for the preceding 1R interval. This method for settingdispersion compensation is performed according to the graph shown inFIG. 4B.

[0062] By suitably setting the amount of dispersion compensation so asto correspond to a preceding 1R interval, as described above, an eyeopening which is wide enough can be obtained as indicated by the lowereye pattern shown in FIG. 5. As a result, the logical values “1” and “0”can be accurately obtained.

[0063] Especially, when a short distance of 10 km first exists, thetransmission characteristic significantly differs depending on thecompensation methods. In this case, a better waveform can be obtainedunder the (lower) compensation condition (2) rather than the (upper)condition (1). That is, the method for determining the amount ofdispersion compensation according to the distance prior to arepeater/receiver is effective.

[0064]FIG. 6 is a schematic diagram showing the number of 1 Rs, whichsatisfy a transmission characteristic required for the amount ofdispersion compensation on a transmitting side, for each a parameter.

[0065]FIG. 6 assumes that the 1R transmission distance is set to 80 km,and the amounts of dispersion compensation within in-line amplifier(s)and on a receiving side are set to −1000 ps/nm. Here, the number of 1 Rsis the number of relays using linear repeaters.

[0066] It can be seen from FIG. 6 that if the a parameter is negative, arequired transmission characteristic can be satisfied for up to only two1 Rs. However, by setting the a parameter positive, this phenomenon canbe improved. Especially, if the a parameter is +1, the requiredtransmission characteristic can be obtained for the widest range, andthe maximum amount of dispersion compensation on the transmitting sidewill be −1200 ps/nm.

[0067] To obtain the required transmission characteristic means that awaveform of a light pulse signal changes up to 10% in the amplitudedirection and up to 30% in the phase direction in comparison with thecase in which no influence is given.

[0068] That is, it is shown from FIG. 6 that a longer transmissiondistance can be secured by which a required transmission characteristiccan be obtained when the a parameter is positive rather than negative.Especially, the longest transmission distance can be secured if thevalue of the a parameter is +1.

[0069] Note that, however, the value of the a parameter which can obtainthe longest transmission distance may vary when the transmission outputof an optical signal is changed. This is because the optimum value ofthe a parameter depends on the optical transmission output. At least, itcan be said from this figure that it is better to set the α parameter toa positive value rather than to a negative value.

[0070]FIG. 7 is a schematic diagram showing the relationship of a 1Rinterval to an amount of 1R residual dispersion.

[0071] This figure assumes that the number of 1 Rs (the number ofrepeaters plus 1, for the receiver) is 3, the value of the a parameteris +1, the optical transmission power is +13 to +14 dBm, the amount ofdispersion compensation on a transmitting side is −600 ps/nm, and theamounts of dispersion compensation within an in-line amplifiers and on areceiving side are 0 to −1200 ps/nm. The amount of 1R residualdispersion (the amount of residual dispersion at 1R intervals) isexamined in the range of the 1R interval from 0 to 80 km based on thisassumption.

[0072] It can be seen from FIG. 7 that a required transmissioncharacteristic can be obtained by setting the amount of 1R residualdispersion to approximately 100 to 400 ps/nm even if the 1R intervalvaries. The number of repeaters is 3 in this figure. However, if thenumber of repeaters is set at 2, a repeater interval is expected to beextended up to 120 km. Therefore, the maximum amount of dispersioncompensation on the receiving side is obtained based on the assumptionthat the repeater interval is 120 km. Assuming that the amount of fiberchromatic dispersion is 20 ps/nm/km in this case, the amount ofdispersion of the 1R interval will be 2400 ps/nm. The maximum amount ofdispersion compensation on the receiving side can be obtained as being−2300 ps/nm by subtracting the minimum amount of 1R residual dispersion100 ps/nm from the above described amount.

[0073] The above described embodiment assumes a transmission speed whichis too great to ignore the non-linear effect that an optical signalundergoes on a transmission path. For example, the speed is 10 Gbps.

[0074] According to any of the above described embodiments, a dispersioncompensator prepared for a receiving side can be combined with a modulehaving the same amount of dispersion compensation. For example, theamounts of dispersion compensation within in-line amplifier(s) and on areceiving side are a multiple of −300 ps/nm such as 0, −300, −600, −900and −1200 ps/nm in the compensation setting graph shown in FIG. 4B.Referring to FIG. 4B, such amounts of dispersion compensation can covera 1R interval of up to 80 km.

[0075] Accordingly, a module having the amount of dispersioncompensation −300 ps/nm may be used as a unit for the dispersioncompensation, and combined with other units so as to obtain a requiredamount of dispersion compensation.

[0076] That is, the amount of dispersion compensation basically must bechanged according to a transmission distance (the amount of dispersionwhich occurs on a transmission path). There is a conventional method formeasuring the amount of dispersion on each transmission path, andsetting the amount of dispersion compensation in order to keep theamount of residual dispersion constant. With this method, however,innumerable types of dispersion-compensators, which must becustom-built, are required. As a result, an economic problem occurs whenthis method is put into practice. There is another conventional methodfor appropriately dividing a transmission distance, determining theamount of dispersion compensation for each divided interval, and settingmenus of a dispersion-compensator. If the number of menus is large,however, the number of types of peripheral parts increases. That is, itis not economical.

[0077] According to the present invention, a minimum unit of the amountof dispersion compensation (for example, −300 ps/nm) is set, and onlyone type is used as the basic unit of dispersion compensation. Modulesrespectively having the amount of dispersion compensation of the minimumunit are connected in order to realize the required amount of dispersioncompensation according to the transmission distance. If such adispersion-compensator is used, it is not necessary to change thedispersion-compensator itself, even if a transmission distance ischanged due to a moving of equipment. It is sufficient only to add orremove a module (or modules). Additionally, since the number of types ofmodules is only 1, is very economical to implement different systems.

[0078] With the above described method, however, there is a possibilitythat the transmission characteristic cannot be secured depending onvarious difficulties such as non-uniformity of fibers, a change of anoutput power, etc. It is effective that a dispersion-compensating modulefor correction (such as a module having the amount of dispersioncompensation 100 ps/nm) is prepared in order to cope with the case inwhich the above described problems should happen, and is added to theother modules in order to make a subtle adjustment.

[0079] There is also the case in which the input/output level of adispersion-compensator is made constant, and the loss of thedispersion-compensator must be within a predetermined range regardlessof the amount of dispersion compensation. For example, there may berestrictions imposed by the input levels of an O/E, a post-amplifier,etc. In such a case, the loss of the dispersion-compensator will be setwithin the required range by additionally using an optical attenuator.Alternatively, it is possible to cause a loss with an intentional shiftof optical axes at the time of a fiber splice. The loss is set even ifthe amount of dispersion compensation is changed. Setting prevents asucceeding device from being influenced.

[0080] As a method for connecting modules, a connection by a splice(fusion of fibers), a connection using a connector etc., can be cited.The module itself may be configured so that it can be attached/detached.

[0081]FIGS. 8A through 8D are schematic diagrams explaining modules of adispersion-compensator. FIGS. 8A and 8B show variations of anarrangement of modules. FIG. 8A shows a variation in which modules arearranged in series or side by side, while FIG. 8B shows a variation inwhich modules are stacked.

[0082]FIGS. 8C and 8D show a connection method in the above cases. FIG.8C shows a method for arranging one of the input and output terminals onone of the opposing sides, and arranging the other of the two terminalson the other of the two sides. FIG. 8D shows the structure in which bothinput and output terminals are arranged on one side. In this case, themodule includes a switching circuit, which detects the insertion of aterminal when another module is connected and opens a once closedportion to the opposing terminals, so that the modules become connected.

[0083]FIGS. 9A and 9B exemplify the structure of an optical switch foruse in a module of a dispersion-compensator.

[0084]FIG. 9A shows the implementation in which the insertion of amodule is detected in the arrangement shown in FIG. 8D. When switches132 and 133 are closed, an optical path is established between A and C.Light is input to an output port 130, and output from an output port131. Alternatively, in this implementation, light may be input to theoutput port 131, and output from the output port 130. Dispersioncompensation is made in a portion “A” of the optical path. A portion “C”of the optical path is a normal path which does not have a dispersioncompensation capability.

[0085] When another module is connected, the output and input ports ofthat module are inserted into module connection detector 135 and 136.The module connection detectors 135 and 136 detect when another modulehas been connected, and send a signal to a module connection detectingsignal processing unit 137. The module connection detecting signalprocessing unit 137 sends a control signal to the switches 132 and 133based on this signal. Based on this control signal, the switches 132 and133 switch the optical path so that light travels through A and B.

[0086] The switches 132 and 133 may be of any type as long as they canswitch an optical path upon receipt of an electric signal. A mechanicalswitch is available on the market.

[0087]FIG. 9B exemplifies the specific structure of the moduleconnection detector.

[0088] The module connection detector is arranged in an adaptor 139attached to a connector 138 of the module. In FIG. 9B, a nail-shapedprojecting portion is arranged as a detector 141. When a connector 140,arranged at the output port of another module, is inserted into theadaptor 139, the projecting portion of the detector 141 moves upward,and electrically connects to and turns on a switch 142, arranged at adifferent location. Switch 142 generates a connection detection signal.The module connection detecting signal processing unit 137 detects thissignal, and switches an optical path within the module.

[0089] A dispersion-compensating fiber can be used as the implementationof dispersion compensator. In addition, various other components areavailable for the dispersion compensator.

[0090]FIGS. 10A through 10C are schematic diagrams showing theimplementations of dispersion compensators other than adispersion-compensating fiber.

[0091]FIG. 10A shows a fiber-grating type dispersion-equalizer.

[0092] Assume that a grating (a cyclic change of a refractive index) 144is provided to a fiber 143, and its cycle is changed by degrees. Iflight is input to the fiber 143, the light is reflected at points whichdiffer depending on wavelength, and returns. After the light, to which adifferent delay time is provided depending on the wavelength, returns,it is extracted using a circulator 145, and dispersion-equalized. If thedirection (side) of the input to the fiber grating is reversed, adispersion characteristic having an opposite sign can be obtained.

[0093]FIG. 10B shows an example of a waveguide typedispersion-equalizer.

[0094] A waveguide 146 is formed, for example, using silicon dioxide(SiO₂) on an Si substrate, and a phase shifter 149 is arranged so thatthe phases of an upper waveguide 147 and a lower waveguide 148 differfrom each other. The component of an input optical signal on a longwavelength side may propagate along the lower part, while the componenton a short wavelength side may propagate along the upper part by meansof phase adjustment made by a phase shifter 149.

[0095] A negative dispersion characteristic can be obtained by makingthe signal propagate along such a waveguide a number of times. Also adispersion characteristic of the opposite sign can be obtained byadjusting a phase. A thin film heater may be used as the phase shifter149.

[0096]FIG. 10C shows a resonator type dispersion-equalizer.

[0097] A total reflecting mirror 151 and a translucent mirror 150 areopposed. If light is input from the translucent mirror 150, only lighthaving a certain wavelength according to the distance between themirrors is multiplex-reflected in between, and resonated. Light which ismultiplex-reflected a certain number of times proportional to afrequency, and has a frequency in the neighborhood of the resonantwavelength, returns. This light is extracted using a circulator, and adelay time which may differ depending on its frequency (wavelength) isprovided and dispersion-equalized. A dispersion characteristic of anopposite direction can be obtained if the light region to be used has afrequency which is either higher or lower than the resonant frequency.

[0098] The tolerance of the amount of dispersion compensation which cansecure a required transmission characteristic can be improved bychirping an optical signal on a transmitting side as red chirping whosea parameter is positive, arranging a dispersion-compensator in atransmitter, adjusting the amount of dispersion compensation of adispersion-compensator (in order to compensate the dispersion of apreceding transmission path) in a repeater, and arranging adispersion-compensator also in a receiver. As a result, the number ofrequired types of devices can be reduced when the types of adispersion-compensator are set according to a transmission distance.

[0099] Furthermore, an optical output can be made higher since thenon-linear effect on a transmission path is cancelled by performing thered chirping on the transmitting side.

What is claimed is:
 1. A method of producing an optical signal,comprising: converting an electric signal to an optical signal;determining the amount of power necessary to transmit the opticalsignal; and pre-chirping the optical signal such that the value of achirping parameter corresponds with the necessary power.
 2. The methodof producing an optical signal according to claim 1, wherein thepre-chirping widens an optical pulse of the optical signal.
 3. Themethod of producing an optical signal according to claim 1, wherein thechirping parameter α is positive.
 4. The method of producing an opticalsignal according to claim 1, further comprising the step of compensatingfor dispersion.